Outbreak of the First World War: 28 July 1914

The Austro-Hungarian government was slow to respond to the assassination of Archduke Franz Ferdinand at the hands of the Serbian-trained terrorist Gavrilo Princip. By the time the Austro-Hungarian government delivered an ultimatum to Serbia, much of the initial shock at the assassination had subsided throughout Europe. Many European nations regarded the ultimatum as an attempt by Austria-Hungary to bully the Serbs into submission. Both Russia and France supported Serbia, which accepted all but two of the provisions of the ultimatum.

On 28 July 1914, the Austro-Hungarian Empire mobilized against Serbia. Russia mobilized against Austria-Hungary, and, on 1 August, Germany declared war on Russia. Italy refused to honour its commitment to the Triple Alliance (an agreement between Italy, Austria-Hungary, and Germany). Italy asserted that the Triple Alliance was a defensive pact and that, since Austria-Hungary had declared war on Serbia, it was not obliged to act. On 3 August, Germany declared war on France. As the German armies advanced through Belgium on 4 August, Britain declared war on Germany for violating Belgian neutrality. The British declaration of war was made on behalf of the dominions as well, and Canada found itself at war with Germany and Austria-Hungary.